In chemistry and biochemistry chemical reactions are frequently performed on the surface of small polymeric resin beads of a diameter range 50-600 microns, typically 250-300 microns in diameter. For example an active substance may be chemically absorbed onto such a bead, e.g. on its surface and/or into the body of the bead, the treated bead may then be exposed to further active substances or labels such as fluorescence labels, and the occurrence of an interaction between the various active substances and/or labels can be detected by monitoring the bead. This technique is particularly useful in biological sciences where only minute amounts of such active substances may be available, and in the investigation of combinatorial chemistry libraries.
In such techniques it is usually necessary to transfer individual beads from a source containing many such beads, e.g. a suspension (which term as used herein includes inter alia a slurry) of the beads in a liquid, into defined locations in a receptacle, for example individual vials, or individual wells in a microtitre plate.
A number of problems hinder such transfer. The small size of the beads makes them difficult to manipulate. It is difficult to transfer beads using present techniques without at the same time transferring a relatively large quantity of liquid. Often large numbers of beads are involved, and present methods of transferring beads suffer from the problems inherent in handling large numbers of small beads. At present either manual methods, e.g. picking out single beads by hand, or excessively complex devices are used.
One device for automatically separating beads from a suspension of a plurality of beads in suspension in a fluid is disclosed in WO 94/28119, which is a large scale device, primarily intended for use with beads of diameter ca. 3 mm, and for separating beads containing encapsulated biomass from empty beads. Devices for automatically separating smaller particles, e.g. cells suspended in a fluid, are disclosed in U.S. Pat. No. 4,756,427, U.S. Pat. No. 5,030,002 and U.S. Pat. No. 4,175,662. It is considered unlikely that the bead sorting devices disclosed in these last mentioned US patents could be adapted directly for use in separating the above mentioned polymeric beads used in chemical reactions. Moreover they all suffer from the disadvantage that the suspended particles remain in a substantial bulk of suspension fluid even after separation. This means that if the particles are to be delivered to a particular location, e.g. a specified well in a microtitre plate, this excess of fluid accompanies the particle to the location.
It is an object of this invention to provide a device which is more suitable for operation with smaller volumes of suspension and for the above-mentioned smaller polymeric beads. It is also an object of this invention to provide a device which can separate beads and can be used with a range of sizes of beads, i.e. not being limited to a particular size of bead.